1. Field of the Invention
Embodiments of the invention described herein pertain to the field of submersible pump assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable an apparatus, system and method for reducing gas to liquid ratios in submersible pump applications.
2. Description of the Related Art
Submersible pump assemblies are typically used to artificially lift fluid to the surface in deep wells such as oil, water or gas wells. A typical electric submersible pump (ESP) assembly is located deep in the ground and, from upstream to downstream, consists of downhole sensors, an electrical motor, seal section, pump intake and pump. The motor, seal, intake and pump are all connected together with shafts that run through the center of the ESP assembly components. The electrical motor supplies torque to the shafts, which provides power to the pump. Production tubing connects the pump to piping or storage tanks at the surface of the well.
Centrifugal pumps are often used in ESP applications for lifting well fluid to the surface. Centrifugal pumps impart energy to a fluid by accelerating the fluid through a rotating impeller paired with a stationary diffuser. The rotation confers angular momentum to the fluid passing through the pump. The angular momentum converts kinetic energy into pressure, thereby raising the pressure on the fluid and lifting it to the surface. Multiple stages of impeller and diffuser pairs may be used to further increase the pressure.
Conventional centrifugal pumps are designed to handle fluid consisting mainly of liquids. However, well fluid often contains gas in addition to liquid, and currently available submersible pump systems are not appropriate for pumping fluid with a high gas to liquid ratio. When pumping gas laden fluid, the gas may separate from the other fluid due to the pressure differential created when the pump is in operation. If there is a sufficiently high gas to liquid ratio (GLR), typically around 10% to 15% gas volume fraction, the pump may experience a decrease in efficiency and decrease in capacity or head (slipping). If gas continues to accumulate on the suction side of the pump impeller, the gas may entirely block the passage of other fluid through the impeller. When this occurs the pump is said to be “gas locked” since proper operation of the pump is impeded by the accumulation of gas. As a result, careful attention to gas management in submersible pump assemblies is needed in order to improve the production of gas laden fluid from subsurface formations.
Currently, attempts are sometimes made to remove gas from produced fluid prior to the fluid's entry into the pump intake. For example, gas separators are sometimes implemented as an additional pump assembly component for this purpose. However it is often infeasible, costly or too time consuming to ascertain the correct type of pump and separator combination which might be effective for a particular well, and even if the correct arrangement is ascertained, the separator may not remove enough gas to prevent a loss in efficiency and/or prevent gas locking. Alternatively, perforations in the well casing are sometimes placed above the pump intake and implemented with a shroud. The shroud forces well fluid deeper into the well before entering the pump intake, a portion of the gas breaking out of the fluid in the process. A drawback to the use of a shroud is that conventional shrouds are prone to leaks. If well fluid were to leak directly into the pump, the fluid entering the well casing above the pump intake would bypass the motor, which would be at risk of overheating or failure due to the lack of cool, fresh flowing fluid passing-by during operation.
The motor of an ESP assembly is conventionally operated using a variable speed drive (VSD), which is controlled by a well operator with a VSD controller user interface located at the surface of the well. Typically, if the GLR becomes too high, this may be detected by the VSD controller since the load on the motor becomes lighter, the motor does not pull as much amperage and the temperature of the motor increases. In response, the pump operator would typically modify the speed of the pump's motor or hold more back pressure on the tubing in an attempt to prevent slipping or gas locking. However, this approach only meets with somewhat limited success, as the high gas to liquid ratio remains in the produced fluid.
Thus, conventional ESP assemblies are not well suited for pumping fluid with a high gas to liquid ratio. Therefore, there is a need for an apparatus, systems and method for reducing gas to liquid ratios in submersible pump applications.